I found that with my Kubota
L3400 with loader and brush hog a 20' was what was really needed. The Ford 1920 w/loader and brush hog will be very close to that size and weight.
I first picked up a used 16'er. Brush hog had to hang off the back some, and there was no room to adjust the weight balance of the tractor on the trailer. I then sold that trailer (for more than I paid for it) and bought the 20'er at the suggestion of the PJ Trailer supplier. Perfect fit for my use. Although the above suggestions are very good they put the price range above what I felt I could afford.
Buying used, your neighbor will probably not have too many choices regarding hitch type, lights, etc., but I suggest a Bulldog hitch and LED lights if possible. Tandem trailer with working brakes are a minimum, preferably on both axles but I think that'll be rare. A 7,000 gross weight trailer will be adequate. Good flooring and good tires would also be on top of my list. He'll be lucky to find a used trailer meeting those specs, and if he does, it'll probably be close in price to a new trailer.
Given that situation, the choice for me would be simple - buy new. If budget doesn't allow that, I hope he has good luck on finding just the right trailer for his needs.